1. Field of the Invention
The present invention relates to a process for producing three-dimensional articles made of intermetallic compounds using a layer by layer technique. The process according to the present invention can be applied, for example, in the motor vehicle and/or aeronautical sector to produce specific components, such as valves for internal combustion engines, gas turbines, blades and/or impellers for turbo compressors or the like.
2. Discussion of Related Art
These types of components must have noteworthy properties of creep and fatigue strength at high temperatures. For these reasons, research is oriented towards various materials, in particular intermetallic compounds, capable of imparting the desired properties to each article produced.
Intermetallics are metal compounds whose crystal structures differ from those of the metals of which they are composed. This is a class of singular compounds that are composed of ordered phases of two or more metal materials whose atomic species occupy specific sites in the structure of the crystal. These compounds are formed because the strength of the bond between different atoms is greater than the one between identical atoms.
Intermetallic compounds differ considerably from conventional metal alloys. The latter are essentially formed of a disordered solid solution of one or more metallic elements, do not have a specific chemical formula and are described as being composed of a base material to which certain percentages of other elements have been added. The atoms of conventional alloys are bonded by relatively weak metallic bonds, with atomic nuclei floating in a “gas” of electrons which move relatively freely.
On the contrary, an intermetallic compound is a specific chemical compound based on a specific chemical formula, with a chemical composition that is fixed or in any case very limited in variability. The bonds in intermetallic compounds can be ionic or covalent, and therefore particularly strong. Alternatively, the bonds can also be entirely of the metal type, but the atoms of the single elements take preferred positions in the crystal structure.
These peculiarities reflect on the properties of intermetallic compounds, such as high melting point, noteworthy resistance to high temperatures but low ductility.
Titanium and aluminium intermetallic compounds, and in particular the compounds defined with the abbreviation γTiAl (γTitanium Aluminides), represent the group of intermetallics of most interest for application in the motor vehicle and aeronautical field due to their properties of low density and high resistance to high temperatures. This group of intermetallics includes γTiAl compounds with tetragonal LI0 ordered structure with centred faces, and compounds defined with the abbreviation α2Ti3Al with hexagonal DO19 ordered structure. In conditions of thermodynamic equilibrium, the γ/α2 volume ratio is controlled on the basis of the aluminium content and of other additional elements, but thermal and/or thermomechanical treatments have a high influence on the γ/α2 volume ratio in γTiAl compounds.
In view of the growing interest in γTiAl intermetallic compounds, studies carried out in the last 10 years have identified the ideal composition ranges that provide for a very similar aluminium and titanium content, between 44% and 48%, together with small percentages of other elements that impart specific properties to the resulting intermetallic compound. For further information on intermetallic compounds see, for example, the following publications:                G. Sauthoff “Intermetallics”, Weinheim, N.Y. (1995); and        H. Clemens, F. Appel, A. Bartels, H. Baur, R. Gerling, V. Guther, H. Kestler, “Processing and application of engineering g-TiAl based alloys”, in Ti-2003 Science and Technology, Volume IV, Wiley-VCH.        
The advantages achieved by the use of γTiAl intermetallic compounds are principally their low density (3.9-4.2 g/cm3 as a function of their composition), high specific fatigue strength, high specific stiffness, considerable resistance to oxidation and considerable creep strength up to high temperatures. Nonetheless, as has been shown, it is difficult to obtain an article produced with a γTiAl intermetallic compound with exactly the composition and structure desired.
A typical example of articles, which can advantageously be produced with a γTiAl intermetallic compound, regard gas turbine blades. Besides the difficulties already mentioned concerning obtaining an article produced with the desired material, it must also be borne in mind that these articles require extremely precise machining operations but are difficult to mass produce at reasonable costs with conventional metallurgical techniques, above all due to their extremely complex geometry. A further difficulty lies in the fact that articles of this type must have an extremely low oxygen content, preferably much lower than 1,500 ppm.
Machining from the solid entails very high costs and is therefore unacceptable for mass production. Other known manufacturing techniques for this type of component are just as unprofitable.
For example, the lost wax casting technique can entail a high number of rejects, above all due to the porosities and cracks that are created in articles thus produced and does not allow particularly complex geometries to be obtained.
Another widely used technique is hot pressing, but besides requiring particularly high temperatures and therefore high energy consumption, it is still difficult to obtain complex geometries without requiring several other machining cycles for each article produced.
Neither of these known techniques is therefore able to guarantee the necessary repeatability in the composition of the material, and therefore of the mechanical properties in mass produced articles. This is an essential requisite, above all for mass production of components for engines and/or turbines, in order to guarantee a high level of reliability.
Alternative manufacturing techniques have recently been added to conventional techniques, such as layer by layer manufacturing techniques, which essentially use a focused beam of laser light to obtain melting and/or sintering of successive sections or layers of materials melted by an incident laser light.
These techniques, known for example as “Direct Laser Forming” (DLF) or “Laser Engineered Net Shaping” (LENS), entail a launching system of the metal powders generally integral with the laser light emitter and simultaneous injection of a jet of inert gas (argon), aligned with the laser beam to protect the melting area from oxidation.
Nonetheless, a high degree of turbulence occurs at the melting point due not only to the jet of inert gas, but also to evaporation of metal powders, above all in the case of aluminium powders. In fact, the temperature in the melting chambers is particularly low, in the order of approximately 100° C., but the metal powders have very high melting temperatures, often over 1000° C. The beam of the laser light loses focus making the melting process difficult and evaporation of part of the chemical species with lower melting temperatures compromises the final composition of the article. Moreover, the jet of argon alone in unable to limit oxidation of the article during manufacture to acceptable values.
Consequently, these known techniques cannot be utilized for mass production, nor can they be used to produce articles made of intermetallic compounds, but at the most are usable to produce metal alloy coatings on articles already manufactured (“Laser Cladding” or “Direct Metal Deposition”).
Further information on these techniques can be found in the following publications:                “Practical considerations and capabilities for laser assisted direct metal deposition”—G. K. Lewis et al.;        “Laser forming titanium components”—D. H. Abbot et al.;        “Producing titanium aerospace components from powder using laser forming”—F. G. Arcella et al.; and        “The optimisation of processing parameters and characterisation of microstructures of direct laser fabricated TiAl alloy components”—D. Srivastava et al.        
Patent application WO-0181031 by the applicant ARCAM AB describes a layer by layer manufacturing technique for producing three-dimensional articles. The machine includes a melting chamber in which there is disposed a movable work table on which the article is formed by successive depositions of layers of powders. Each layer of powders laid down in the chamber is melted using a beam of electrons according to the technology known as “Electron Beam Melting” (EBM).
In practice, after having laid down a layer of powders, the emission source of the electron beam is activated to fuse the powders only at one cross section of the three-dimensional article to be formed. Using EBM technology, the machine necessarily operates under high vacuum conditions, thereby reducing the risk of oxidation in the material of the article. This document suggests in particular controlling the temperature in the melting area and modulating the energy of the incident electron beam to prevent vaporization of the melted material.
However, there are no suggestions for particular measures to be taken for the manufacture of articles made of particularly critical materials, such as intermetallic compounds and, in particular γTiAl intermetallic compounds.